Air impact sandbox moulding machines exist that use a release valve located between the bell and the diffuser, comprising a rigid disc or plate that is kept in the closed position by action of the overpressure existing in a chamber in which said plate plays, and hence upon the overpressure in such chamber disappearing, the accumulated air in the bell is capable of displacing the same to the open position, brusquely entering the diffuser and hitting the sand. Upon air under a greater pressure being again introduced in said chamber, in which the release valve plays, the latter moves back towards the closed position.
This solution results in brusque impacts and consequently noises when the closed position changes to the open position, but most importantly, after the release, the residual air has to be delivered through side ducts provided in the frame or close to the mouth of the diffuser, and thus its delivery causes sand particles to be dragged that significantly damage the machine because they are extremely abrasive.
Another known solution consists of using butterfly valves which, being of the mechanic kind, have a slow opening or closing response and the residual air must, as in the previous case, be removed through side ducts provided in the frame, with the same problems aforesaid.
A more advanced solution is described in a patent. It consists of using a highly elastic membrane as a delivery valve, solidly attached by means of two annular concentric flanges to a supporting plate within the bell body, thereby for a tightly sealed annular chamber to be configured between the membrane and its support, to which a compressed air supply duct has access, which membrane is operatively arranged facing a narrow annular seat and thus, when idle, an annular passage is established between the membrane and the seat, whereas when said annular chamber is applied pressure, the membrane is deformed and sealed tightly against its seat.
The main drawback of this solution, which it shares with the preceding cases, is a deficient compacting about the outer mould contour, or in other words, the inner moulding box contour. Deflectors are generally used to solve this problem, directing the air current towards the comers or edges of the moulding box, but this results in a substantial efficiency loss and further demands that very high impact pressures be used to achieve an effective compacting on the edges of the box.
Another solution to improve peripheral mould compacting is described in another patent. In it the air impact moulding machine is provided with a sand-supplying hopper axially arranged within the air release bell, and therefore both elements have direct access into the moulding chamber, the air outlet area from the bell being provided with a tube-shaped membrane valve that is axially deformed to open or close the passage of compacting air from the bell towards the sand mould. In addition to being more complicated due to the inclusion of the sand loading hopper at its central area, this machine moreover fully prevents the use of ancillary compacting systems, such as hydraulic rams for instance.